1. Technical Field
The present disclosure relates to multi-color printing systems, and, in particular, to a system and method for characterizing misregistration between color separations in a multi-color printing system by utilizing a misregistration estimation patch formed by frequency-shifted halftone patterns that form a moiré pattern.
2. Description of Related Art
In most multi-color printing systems, such as xerographic color printers, multiple color separations are used for marking a substrate, e.g. paper. Usually each separation marks the substrate with only one specific colorant, which is different from colorants from other separations. The common combination of color separations are cyan, magenta, yellow and black, also referred to as CMYK. A separation can utilize “ink” and/or “toner” to mark a substrate, and for the purposes of the disclosed subject matter, the two terms can be used interchangeably.
It is well understood that most color printers operate in a binary mode, i.e., for each color separation, a corresponding color spot is either printed or not printed at a specified location or pixel, and halftone techniques control the printing of color spots. Spatially averaging the printed color spots of all the color separations by a human visual system provides the illusion of the required continuous color tones. The most common halftone method is screening, which compares the required continuous tone levels with predetermined threshold levels typically defined for a rectangular cell, or a halftone screen, that is tiled to fill the image plane. The output of the screening process is a binary pattern of multiple small “dots,” which are regularly spaced as is determined by the size, shape, and tiling of the halftone screen. In other words, the screening output, as a two-dimensionally (2-D) repeated halftone pattern, possesses two fundamental spatial frequencies, which are completely defined by the geometry of the halftone screen.
Multi-color printing systems are susceptible to misregistration between color separations due to a variety of mechanical related issues. For example, the separations may be orientated differently in one direction or another due to the mechanical tolerances of the separations; also, vibration may create localized misregistration by moving slightly a separation in an undesirable fashion for a short time. Color separation misregistration may cause a significant color shift in the actual printed color that is noticeable to the human eye. Additionally, an unintentional “beating” pattern, or moiré pattern, may appear when viewing a printed image with color separation misregistration.
Moiré patterns are undesirable interference patterns that happen when two or more color halftone separations are printed over each other. Since color mixing during the printing process is a non-linear process, frequency components other than the frequencies of the individual color halftone separations can occur in the final printout. As a result, low frequency components might be visibly evident as pronounced moiré interference patterns in the halftone output. To avoid color moiré, different halftone screens are commonly used for different color separations, where the spatial directions of halftone patterns of different colors are separated by relatively large angles. Therefore, the frequency difference between any two frequency components of the different screens will be large enough so that no visibly objectionable moiré patterns are produced.
When using rotated halftone screens, the resulting halftone outputs are more robust to misregistration between different color separations. However, even in these cases, separation misregistration may be objectionable, particularly at the edges of texts or objects that contain more than one color. Therefore, it is important to characterize color separation misregistration in order to perform corrective action of these and other anomalies.
Various techniques have been used to attempt to estimate and/or characterize misregistration, such as using physical registration marks. In this approach, a digital file is created by placing vertically oriented lines of color separation A and color separation B, such that the head of the line corresponding to color separation B begins at the tail of color separation A. For an ideal printing device, this digital image would be perfectly replicated; however, for most real printing systems this is not the case, and misregistration between the two color separations A and B (in a direction perpendicular to the axis of the lines) will result in a visible displacement between the two lines in the horizontal direction. Using a flatbed scanner to scan the printed page and simple centroid analysis enables the estimation of misregistration at the location of the lines, in the direction perpendicular to the line axis. Sometimes, these physical registration marks are printed in the corner of the substrate so that microscopic (manual) examination may be facilitated. The same procedure can be repeated for lines oriented in the horizontal direction, and this can be used to measure misregistration in the vertical direction. With the printer speeds and smaller cluster dot sizes now possible there is a need to estimate and characterize misregistration between separations to mitigate or eliminate unwanted artifacts such as moiré patterns, color shifts and/or anomalies at color boundaries.